I wish to present briefly a progress report on the present status of our national horizontal surveys, using for purposes of illustration a lantern slide which represents the first order control established by the United States Coast and Geodetic Survey and other organizations.
It is expected that by next autumn the Coast and Geodetic Survey will have completed two large projects upon which it has been engaged for a number of years. This diagram shows the system of first order triangulation and traverse as we expect it to be at the end of the coming field season. The first large project, a co‐operative one between the Coast and Geodetic Survey and the Geodetic Survey of Canada, is the execution of a belt of first order control spanning the international boundary and extending from Lake Superior to the Pacific Ocean. The Geodetic Survey of Canada is executing the section from the 109th meridian to the Lake of the Woods, and the Coast and Geodetic Survey the remainder. This first order control is connected at frequent intervals with the third order triangulation of the International Boundary so that the boundary surveys will be strongly tied in to the basic surveys of the two countries. It has taken four years to complete this project, because of the very difficult conditions of the terrain. The western end crosses very rugged country where transportation of the field parties is by pack train only. Along part of the northern Minnesota boundary the transportation was by canoes and back‐packing, while on the section along Rainy Lake and Lake Namakan, first order traverse was run over the ice in mid‐winter, with an average temperature while taping of −5° F.
The work was undertaken in response to a request by Dr. A. A. Michelson that the Coast and Geodetic Survey determine the length of the line, over 20 miles in length, between Mount Wilson and San Antonio Mountain, with an accuracy represented by an actual error of not to exceed 1/250,000. To obtain the length of this line, which extended over very rugged country, it was necessary to measure a base in the valley lying to the south, and to carry the base length by triangulation to the line desired.
Before beginning the field work, the members of the Division of Geodesy of the Survey examined critically the modern methods of base measurement and triangulation. In the triangulation the choice of figure, the probable error of a direction, and the method of determining the differences of elevations of the stations were examined with special care. The figure selected gives the greatest possible strength in the connection between the base and the Michelson line. Field methods were adopted which would insure a probable error of a direction of not more than about 0″2 of arc, or about half that obtained in ordinary precise triangulation, and would give the differences of elevation with an error of not more than one foot.
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